Self-assembly of colloidal particles is of great interest and importance due to its potential applications in many fields of use, such as for example, biomaterials, catalytic supports, atomic/molecular phase behavior study and photonics. The ability to design and assemble 3-dimensional structures from colloidal particles is limited by the absence of specific directional bonds. As a result, complex or low-coordination structures, common in atomic and molecular systems, are rare in the colloidal domain.
The past decade has seen an explosion in the kinds of colloidal particles that can be synthesized. A wide variety of new shapes have been made, from rods and cubes to clusters of spheres and dimpled particles and also other types of anisotropic particles including Janus particles, branched particle, triangles and polyhedrons. The self-assembly of such building blocks is largely controlled by their geometry, and thus, only a few relatively simple crystals have been made: face-centered and body-centered cubic crystals and variants. Colloidal alloys increase the diversity of structures, but many structures remain difficult or impossible to make. For example, the diamond lattice, predicted more than 20 years ago to have a full 3-dimensional photonic band gap, still cannot be made by colloidal self-assembly because it requires fourfold coordination. Without directional bonds, such low-coordination states are unstable.
In contrast to colloids, atoms and molecules control their assembly and packing through valence. In molecules like methane (CH4), the valence orbitals of the carbon atom adopt sp3 hybridization and form four equivalent C—H bonds in a tetrahedral arrangement. In the colloidal domain, the kinds of structures that could be made would vastly increase if particles with controlled symmetries and highly directional interactions were available. Consequently, what is needed are colloids with a form of “valence” characteristic which would advantageously resolve a wide variety of commercial needs heretofore not met.